Discovery lands to conclude STS-124 – OMS controller plan

Veteran orbiter Discovery and her crew of seven have returned to Earth on the first opportunity, landing successfully at the Kennedy Space Center for a nominal EOM (End Of Mission) for STS-124.

Such as been the issue-free flight for Discovery, engineers on the ground even had time to simulate data on the future potential of an exaggerated scenario relating to the left OMS (Orbital Maneuvering System) engine.

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Discovery was able to take the first opportunity to land at the Shuttle Landing Facility (SLF) at 11:15am EDT on the first attempt. A second opportunity was available, which would have resulted with wheels on the ground at 12:50pm.

Ahead of her re-entry, the crew closed Discovery’s payload bay doors – which were observed closely, following a fleet wide issue with the radiator retract hoses – that transport freon coolant to the orbiter’s radiators on orbit – suffering from C-bend/Omega-bend issues both at the pad and in orbit.

‘Crew procedure incorporated into Deorbit Prep checklist to capture imagery of all four Freon radiator hoses during payload bay door closure,’ noted one Flight Readiness Review presentation ahead of the mission. ‘Per Shuttle Program, this is a highly desired requirement.’

Had one of the four hoses suffer from a bend, a plan was already in place to isolate the hose in question. This would have protected against any leakage of the freon, and allow Discovery to target a nominal re-entry plan.

In what in believed to be a record low, Discovery’s mission only generated nine issues worthy of note by the Mission Evaluation Room (MER), and some of those issues – classed as ‘funnies’ – related to minor problems such as a microphone on a headset. None of the issues had any mission impacts.

One item on the list gained additional attention, despite its lack of impact to Discovery’s current mission.

Left OMS Controller EOM Plan:

That issue – with the Left OMS Secondary TVC (or gimbal) controller’s ‘failure to null in both pitch and yaw’ – saw engineers work scenarios on the ground, for the purpose of additional understanding, in the event of a problematic issue occurring in the future.

The Left OMS engine is currently performing without issue via its working primary controller, and will be used – as per normal – in tandem with the Right OMS engine for the deorbit burn.

‘Use Primary controller to position engine during deorbit burn and entry stow,’ noted one of several presentations on the engine. ‘No reason to believe the primary will fail – it is working now and will be on for the remainder of the flight.’

However, the opportunity to work scenarios – where failed controller gimballed the engine bell/nozzle to the extreme position of ‘actuator hard stop’ – leaving part of the bell exposed to some of the aeroheating of re-entry – was taken by engineers, mainly in thanks to the lack of any issues with Discovery during her mission.

Interestingly, even in the event of such a scenario, the findings revealed that the Columbium engine bell would still avoid any damage, along with no controllability issues.

‘If the engine is in a bad config (scenario): An estimate of aero loads does not indicate controllability issues given the size of the engine bell and the amount it could extend into the airflow (<4 sq ft pitch; <1 sq ft yaw) as compared to the rudder/speedbrake panels and body flap.

‘Aeroheating of the Columbium engine bell during entry (498-771 degrees F) is predicted to be well within the temperature range of the material (<1600 degrees F at flange).’

Engineers also evaluated any potential concern relating to a scenario of possible movement of the OMS engine nozzle during re-entry. Again, the engine’s bell would be under no threat of damage.

‘No concerns from Orbiter Entry Aeroheating team due to possible movement of the OMS engine nozzle into the flow during re-entry,’ added one presentation. ‘Orbiter entry orientation (40 deg angle of attack) shields leeside components from high heating.

‘OMS pod nozzle is further shielded by OMS pod from incoming flow potential protuberance heating caused by nozzle worst-case orientation will be negligible due to thick boundary layer and leeside flow behavior.’

In fact, the worst case scenario for the OMS engine bell – if slightly exposed via a large scale controller issue – would be via the high velocity airstream the orbiter enters during her return, with the potential that the exposed edge of the bell could buckle.

Even then, the orbiter flight’s would not be compromised, with the only requirement being its replacement, once she was back in her Orbiter Processing Facility (OPF).

At present, the plan for Discovery once she lands relates to the post-landing checklist. The OMS engines will remain in their landing configuration until she arrives for post flight deservicing in the OPF, where engineers will work on the root cause of the secondary controller failure.

‘Post-landing ops are minimal: OMS Enables are inhibited at OPS 9 transition; zero position measurement may generate an error during SSME (Space Shuttle Main Engines) Rain Drain repositioning, but will not affect operation.’

‘No OMS motion is planned until the orbiter gets to the OPF except for a possible ferry flight.’

That mission will see her deliver the fourth and final starboard Integrated Truss Segment (S6), with the fourth set of solar arrays and batteries to the International Space Station. A crew rotation on Expedition 18 will also be carried out, with Sandra Magnus replaced by STS-119’s Koichi Wakata.

During her processing flow, Discovery will receive an upgrade to the Inertia Reels on the crew’s launch and entry seats, along with three additional hardware modifications, which are currently in the process of review by shuttle managers.